1. Field of the Invention
The present invention relates to hydraulic coupling assemblies for motor vehicles, and more particularly to a torque-coupling device for an on-demand all-wheel drive (AWD) system having an electronically controlled hydraulic actuator including an electro-magnet actuated variable pressure relief valve for selectively activating a secondary drive axle of the AWD motor vehicle.
2. Description of the Prior Art
Many modern vehicles employ four-wheel drive systems. These systems have been marketed in two forms. Systems generally termed four-wheel drive (4WD) have a transfer case, which is controlled by the operator to select two wheel or four-wheel drive. If the operator selects the four-wheel drive condition, the vehicle drives all four wheels continuously. Some of these systems have employed overrunning clutches at two of the wheels to alleviate some of the disadvantages of 4WD which result from tire pressure differential and cornering to name a few.
All wheel drive (AWD) systems also provide the benefits of a four-wheel drive vehicle and do not require the operator to intentionally select this condition. These systems often employ a viscous clutch in the center differential to transfer torque to the drive wheels that are not sensed as slipping. In tight cornering situations and during towing, these AWD systems present a disadvantage. In cornering situations, noise and vibration can result from the AWD system being engaged. While this is not detrimental to the powertrain during short durations, it can be disconcerting to the operator.
Hydraulic couplings are used in various vehicular drivetrain applications to limit slip and transfer drive torque between a pair of rotary members. In all-wheel drive applications, hydraulic couplings are used to automatically control the drive torque transferred from a driven member to a non-driven member in response to speed differentiation therebetween. In limited slip applications, couplings are used in association with a differential to automatically limit slip and bias the torque distribution between a pair of rotary members.
Such hydraulic couplings conventionally use a frictional clutch between the rotary members. The frictional clutch may be selectively actuated by various hydraulic actuator assemblies, which are constructed of elements disposed inside the differential casing. The hydraulic actuator assemblies internal to the differential case often include displacement pumps disposed inside the differential casing and actuated in response to a relative rotation between the differential case and the output shaft. The displacement pumps are usually in the form of internal gear pumps, such as gerotor pumps adapted to convert rotational work to hydraulic work. In the internal gear pumps, an inner gear having outwardly directed teeth cooperates with an external gear having inwardly directed teeth so that fluid chambers therebetween increase and decrease in volume as the inner and outer gears rotate in a housing.
By connecting the inlet and outlet of the device to the proper location along the sides of the gear set, the variable displacement chambers receive and discharge hydraulic fluid so that the device can function as a pump or motor. A shaft or other mechanical device can be connected to either the inner or outer gear depending upon the type of device. The hydraulic actuator assemblies further include a hydraulic piston member for frictionally loading the friction clutch.
While known hydraulic couplings, including but not limited to those discussed above, have proven to be acceptable for various vehicular driveline applications, such devices are nevertheless susceptible to improvements that may enhance their performance and cost. With this in mind, a need exists to develop improved hydraulic couplings and driveline apparatuses that advance the art.
Moreover, there is a problem with the current hydraulic coupling in that they do not have a simple on/off capability, which is separate and distinct from the hydraulic pressure supply/control circuit actuating the clutch assemblies. Therefore, it is the intent of this invention to overcome these shortcomings by providing an external control of the hydraulic pressure generated within a hydraulically actuated limited slip coupling in which the limited slip clutch can either be turned on or off, or set at any intermediate condition by controlling the maximum system hydraulic pressure limit.
The present invention provides an improved torque-coupling device for a drivetrain of an all wheel drive (AWD) motor vehicle including an internal combustion engine coupled to a transaxle of a primary full-time drive axle assembly, a power transfer unit, a propeller shaft transmitting engine torque to a selectively operable secondary, on-demand drive axle assembly, and the torque-coupling device for selectively, on demand activating of the secondary drive axle assembly of the all-wheel drive motor vehicle and providing an infinitely variable torque distribution between the primary and secondary axle assemblies of the AWD motor vehicle.
The torque-coupling device in accordance with the present invention comprises a hollow housing, an input shaft and an output shaft both rotatably supported within the housing, a hydraulically operated, selectively engageable friction clutch assembly for frictionally coupling the input shaft to the output shaft, and a hydraulic clutch actuator. The hydraulic actuator includes a speed sensitive, positive displacement hydraulic pump located within the casing and adapted to generate a hydraulic pressure to frictionally load the friction clutch assembly, a piston assembly for axially loading the clutch pack, and a variable pressure relief valve assembly fluidly communicating with the hydraulic pump to selectively control the hydraulic discharge pressure generated by the pump. The friction clutch assembly and the hydraulic pump are disposed within a clutch casing drivingly coupled to the output shaft. Alternatively, the clutch casing may be drivingly coupled to the input shaft.
The variable pressure relief valve assembly includes a valve closure member, a valve seat complementary to said valve closure member, and an electro-magnetic actuator adapted for selectively varying a release pressure of the pressure relief valve assembly between a maximum release pressure when the friction clutch pack is in the fully xe2x80x9cONxe2x80x9d condition, and a minimum release pressure when the friction clutch pack is in the fully xe2x80x9cOFFxe2x80x9d condition based on a magnitude of an electric current applied to the electro-magnetic actuator.
The electro-magnetic actuator includes a coil winding supported by the clutch casing and an armature radially spaced from said coil winding and axially movable relative thereto in response to a magnetic flux generated by said coil winding when said electrical current is supplied thereto, said armature engages said valve closure member and urges thereof against said valve seat with an axial force determined by said magnitude of said electric current for selectively setting up said release pressure of said valve closure member.
The variable pressure relief valve assembly is operated by an electro-magnetic (preferably, solenoid) actuator electronically controlled by a coupling control module (CCM) based on one or more vehicle parameters as control inputs, such as a vehicle speed, a wheel speed difference, vehicle yaw rate, a vehicle lateral acceleration, a steering angle, an engine throttle position, a brake application, an ice detection, a moisture detection, a vehicle driveline configuration, a vehicle yaw stability control system and an anti-lock brake system/traction control system (ABS/TCS). When energized, the solenoid-operated valve assembly is capable of modulating a pump discharge pressure in a variable range from a minimum pressure to a maximum pressure, thereby selectively and variably controlling a drive torque applied to the output axle shafts in a range from a minimum torque value to a maximum torque value.
In accordance with the first exemplary embodiment of the present invention, the housing of the torque-coupling device of the present invention is secured to an axle housing of the secondary drive axle assembly, an input shaft is drivingly coupled to the propeller shaft of the AWD motor vehicle, and the output shaft is, preferably, in the form of a pinion shaft of the secondary drive axle assembly.
In accordance with the second exemplary embodiment of the present invention, the housing of the torque-coupling device of the present invention is secured to a housing of the power transfer unit, the input shaft is in the form of an output shaft of the power transfer unit, and the output shaft drivingly coupled to the propeller shaft.
Therefore, the selectively operable, secondary drive axle torque-coupling device for the AWD motor vehicles in accordance with the present invention represents a novel arrangement of the hydraulically actuated AWD torque-coupling device provided with an electro-magnetic actuator for activating a variable pressure relief valve for allowing selective actuation of the secondary drive axle and infinitely variable torque distribution between the primary and secondary drive axles of the AWD motor vehicle.